The require_activation parameter is set by the WP Discourse plugin if a site isn’t using the default WordPress registration system. When it is set, WordPress users are not matched to Discourse users by their email address. Existing users who already have an SSO record associated with them should have no problem logging in though. New users should also be able to create accounts without any problem.
Are all SSO logins failing on the site? Are you able to try creating a new account on WordPress and then logging into Discourse via SSO?
You might find some useful information here: Debugging and fixing common SSO issues.